tmac/tmac.an: define .MR in a groff compatible way

groff 1.23.0 added .MR to its -man macro package.  The NEWS file states
that the inclusion of the macro "was prompted by its introduction to
Plan 9 from User Space's troff in August 2020."  From d32deab it seems
that the name for Plan 9 from User Space's implementation was suggested
by groff maintainer G. Brandon Robinson.

Not sure if the intention was to make these definitions compatible, but
it would be nice if they were.

Currently, Plan 9 from User Space's .MR expects its second argument to
be parenthesized.  groff's .MR does not.  This results in extra
parentheses appearing in manual references when viewing Plan 9 from User
Space's manual pages on a system using groff.

man/man1/sam.1

@@ -51,7 +51,7 @@ copies leading white space on the current line to the new line.
 Do not `download' the terminal part of
 .IR sam .
 Editing will be done with the command language only, as in
-.MR ed (1) .
+.MR ed 1 .
 .TP
 .BI -r " machine
 Run the host part remotely
@@ -69,7 +69,7 @@ for debugging.
 .PD
 .SS Regular expressions
 Regular expressions are as in
-.MR regexp (7)
+.MR regexp 7
 with the addition of
 .BR \en
 to represent newlines.
@@ -698,7 +698,7 @@ There is usually a `current window',
 marked with a dark border, to which typed text and editing
 commands apply.
 Text may be typed and edited as in
-.MR rio (1) ;
+.MR rio 1 ;
 also the escape key (ESC) selects (sets dot to) text typed
 since the last mouse button hit.
 .PP
@@ -798,7 +798,7 @@ the white-space-delimited block of text is sent as a plumb message
 with a
 .B click
 attribute defining where the selection lies (see
-.MR plumb (7) ).
+.MR plumb 7 ).
 .TP
 .B look
 Search forward for the next occurrence of the literal text in dot.
@@ -909,11 +909,11 @@ source for the separate terminal part
 .TP
 .B \*9/bin/E
 .SH SEE ALSO
-.MR ed (1) ,
-.MR sed (1) ,
-.MR grep (1) ,
-.MR rio (1) ,
-.MR regexp (7) .
+.MR ed 1 ,
+.MR sed 1 ,
+.MR grep 1 ,
+.MR rio 1 ,
+.MR regexp 7 .
 .PP
 Rob Pike,
 ``The text editor sam''.